The invention is directed to a method for the production of rotors for screw-type compressors, and rotors to be produced by said method.
For the production of rotors for a screw-type compressor, use is made--besides machining production techniques--of master mold techniques wherein the rotor is produced by filling a negative mold with a suitable rotor material. The terms "master mold techniques" or "master molding" are used to define a method wherein a first mold is produced from loose material by generating the required cohesion of material (primary shaping). An example of a master mold technique is a casting process wherein a master mold is filled with material. A master mold technique specially adapted to the production of rotors is known from DE 40 35 534 A1. According to this method, rotors for screw-type compressors are produced from fiber-reinforced synthetic material by stacking disks of such fiber-reinforced synthetic material upon each other in the hollow cavity of a negative mold and connecting them to each other by application of heat and pressure.
Also the negative mold is generated by a master mold technique wherein a representation is produced of the contour of a master rotor, the master rotor corresponding in shape to the rotor to be produced.
Due to the double use of a master mold technique, the shape of the rotor to be inserted into the screw-type compressor is determined by the master rotor. The master rotor is usually produced by a machining technique. However, the tools used for producing the master rotor, e.g. slotting or grinding machines, which generate the three-dimensionally curved surface of the teeth, require cutout portions for the application of the tools. For forming the tooth-root regions, it is up to now required that rounded deepened portions are worked by use of a tool into cylindrical outer surfaces extending concentrically to the rotor axis. These rounded deepened portions are necessary to allow the application of the tool used for working on the tooth flanks.
When a rotor comprising such rounded deepened portions is to be inserted into a screw-type compressor together with a second rotor, the second rotor must have the tips of its teeth provided with edges which engage the rounded deepened portions during meshing. On the other hand, it is not possible to effect a sufficient sealing between the rotors. However, the edges of the second rotor enlarge its diameter and thus the diameter of the housing portion enclosing the second rotor. The enlargement of the diameter of said housing portion and the rounded portions of the edges cause a deterioration of the sealing properties in the engagement region of the two rotors. Before the teeth of the rotors during the rolling movement in the respective cross section come into mutual abutment, a so-called blow-hole lies therebetween, allowing compressed gases flowing back therethrough to the low pressure side.
Thus, the rotor geometry due to the manufacturing conditions will result in backflow losses which cause low efficiency in known screw-type compressors.